security: allow to block desktopCapturer.getSources() calls (#15964)

* security: allow to block desktopCapturer.getSources() calls

* return empty instead of error

* fix: release resources of DesktopCapturer on exit

atom/browser/api/atom_api_desktop_capturer.h

@@ -9,7 +9,7 @@
 #include <string>
 #include <vector>
 
-#include "atom/browser/api/event_emitter.h"
+#include "atom/browser/api/trackable_object.h"
 #include "chrome/browser/media/webrtc/desktop_media_list_observer.h"
 #include "chrome/browser/media/webrtc/native_desktop_media_list.h"
 #include "native_mate/handle.h"
@@ -18,7 +18,7 @@ namespace atom {
 
 namespace api {
 
-class DesktopCapturer : public mate::EventEmitter<DesktopCapturer>,
+class DesktopCapturer : public mate::TrackableObject<DesktopCapturer>,
                         public DesktopMediaListObserver {
  public:
   struct Source {

docs/api/app.md

@@ -401,6 +401,16 @@ gets emitted.
 **Note:** Extra command line arguments might be added by Chromium,
 such as `--original-process-start-time`.
 
+### Event: 'desktop-capturer-get-sources'
+
+Returns:
+
+* `event` Event
+* `webContents` [WebContents](web-contents.md)
+
+Emitted when `desktopCapturer.getSources()` is called in the renderer process of `webContents`.
+Calling `event.preventDefault()` will make it return empty sources.
+
 ### Event: 'remote-require'
 
 Returns:

docs/api/web-contents.md

@@ -663,6 +663,15 @@ Returns:
 Emitted when the associated window logs a console message. Will not be emitted
 for windows with *offscreen rendering* enabled.
 
+#### Event: 'desktop-capturer-get-sources'
+
+Returns:
+
+* `event` Event
+
+Emitted when `desktopCapturer.getSources()` is called in the renderer process.
+Calling `event.preventDefault()` will make it return empty sources.
+
 #### Event: 'remote-require'
 
 Returns:

lib/browser/api/web-contents.js

@@ -377,6 +377,10 @@ WebContents.prototype._init = function () {
     })
   })
 
+  this.on('desktop-capturer-get-sources', (event, ...args) => {
+    app.emit('desktop-capturer-get-sources', event, this, ...args)
+  })
+
   this.on('remote-require', (event, ...args) => {
     app.emit('remote-require', event, this, ...args)
   })

lib/browser/desktop-capturer.js

@@ -1,7 +1,9 @@
 'use strict'
 
 const ipcMain = require('@electron/internal/browser/ipc-main-internal')
+
 const { desktopCapturer } = process.atomBinding('desktop_capturer')
+const eventBinding = process.atomBinding('event')
 
 const deepEqual = (a, b) => JSON.stringify(a) === JSON.stringify(b)
 
@@ -12,6 +14,14 @@ const electronSources = 'ELECTRON_BROWSER_DESKTOP_CAPTURER_GET_SOURCES'
 const capturerResult = (id) => `ELECTRON_RENDERER_DESKTOP_CAPTURER_RESULT_${id}`
 
 ipcMain.on(electronSources, (event, captureWindow, captureScreen, thumbnailSize, fetchWindowIcons, id) => {
+  const customEvent = eventBinding.createWithSender(event.sender)
+  event.sender.emit('desktop-capturer-get-sources', customEvent)
+
+  if (customEvent.defaultPrevented) {
+    event.sender._sendInternal(capturerResult(id), [])
+    return
+  }
+
   const request = {
     id,
     options: {

lib/renderer/api/desktop-capturer.js

@@ -17,6 +17,16 @@ function isValid (options) {
   return Array.isArray(types)
 }
 
+function mapSources (sources) {
+  return sources.map(source => ({
+    id: source.id,
+    name: source.name,
+    thumbnail: nativeImage.createFromDataURL(source.thumbnail),
+    display_id: source.display_id,
+    appIcon: source.appIcon ? nativeImage.createFromDataURL(source.appIcon) : null
+  }))
+}
+
 exports.getSources = function (options, callback) {
   if (!isValid(options)) return callback(new Error('Invalid options'))
   const captureWindow = includes.call(options.types, 'window')
@@ -35,18 +45,10 @@ exports.getSources = function (options, callback) {
   const id = incrementId()
   ipcRenderer.send('ELECTRON_BROWSER_DESKTOP_CAPTURER_GET_SOURCES', captureWindow, captureScreen, options.thumbnailSize, options.fetchWindowIcons, id)
   return ipcRenderer.once(`ELECTRON_RENDERER_DESKTOP_CAPTURER_RESULT_${id}`, (event, sources) => {
-    callback(null, (() => {
-      const results = []
-      sources.forEach(source => {
-        results.push({
-          id: source.id,
-          name: source.name,
-          thumbnail: nativeImage.createFromDataURL(source.thumbnail),
-          display_id: source.display_id,
-          appIcon: source.appIcon ? nativeImage.createFromDataURL(source.appIcon) : null
-        })
-      })
-      return results
-    })())
+    try {
+      callback(null, mapSources(sources))
+    } catch (error) {
+      callback(error)
+    }
   })
 }

spec/api-app-spec.js

@@ -370,6 +370,16 @@ describe('app module', () => {
       w = new BrowserWindow({ show: false })
     })
 
+    it('should emit desktop-capturer-get-sources event when desktopCapturer.getSources() is invoked', (done) => {
+      app.once('desktop-capturer-get-sources', (event, webContents) => {
+        expect(webContents).to.equal(w.webContents)
+        done()
+      })
+      w = new BrowserWindow({ show: false })
+      w.loadURL('about:blank')
+      w.webContents.executeJavaScript(`require('electron').desktopCapturer.getSources({ types: ['screen'] }, () => {})`)
+    })
+
     it('should emit remote-require event when remote.require() is invoked', (done) => {
       app.once('remote-require', (event, webContents, moduleName) => {
         expect(webContents).to.equal(w.webContents)

spec/api-desktop-capturer-spec.js

@@ -1,6 +1,6 @@
 const chai = require('chai')
 const dirtyChai = require('dirty-chai')
-const { desktopCapturer, remote } = require('electron')
+const { desktopCapturer, ipcRenderer, remote } = require('electron')
 const { screen } = remote
 const features = process.atomBinding('features')
 
@@ -99,5 +99,14 @@ describe('desktopCapturer', () => {
       }
       done()
     })
+
+    it('returns empty sources when blocked', done => {
+      ipcRenderer.send('handle-next-desktop-capturer-get-sources')
+      desktopCapturer.getSources({ types: ['screen'] }, (error, sources) => {
+        expect(error).to.be.null()
+        expect(sources).to.be.empty()
+        done()
+      })
+    })
   })
 })

spec/static/main.js

@@ -233,6 +233,12 @@ app.on('ready', function () {
   })
 })
 
+ipcMain.on('handle-next-desktop-capturer-get-sources', function (event) {
+  event.sender.once('desktop-capturer-get-sources', (event) => {
+    event.preventDefault()
+  })
+})
+
 ipcMain.on('handle-next-remote-require', function (event, modulesMap = {}) {
   event.sender.once('remote-require', (event, moduleName) => {
     event.preventDefault()